RICHARD C. BENSON, NEWMAN deHAAS, PHILLIP G. GOODWIN, and TERRY E. PHILLIPS

EPOXY ADHESIVES IN MICROELECTRONIC HYBRID APPLICATIONS

The advantages of using epoxy adhesives in the fabrication of hybrid microelectronic devices are unfortunately offset by their tendency to release volatile materials over time into the interior of the device package, which subsequently can affect the device's long-term reliability. Our work on a model epoxy system focuses on identifying and understanding the source and production mechanism of these impurities, and aims to eliminate them.

INTRODUCTION

In many military, space, and other high-reliability applications, instrumentation packages often incorporate microelectronic hybrid devices. These devices , which perform a single function or related multiple functions , constitute a physical package that is usually hermetically sealed. The package can be considered as a building block in the structure of the overall system design. It provides physical protection and isolation of the internal elements, as well as electrical connections from these elements to the outside world, that is , integration with the other elements of the system.

Within the hybrid package, one finds a lower level of integration in which electronic components (chips), typ-ically silicon or gallium arsenide, operate with functions ranging from those of simple resistors , capacitors, di-odes, and transistors, to the more complex analog func-tions (e.g., amplifiers, multipliers, and mixers) and digital functions (e.g. , logic gates, memory, and processors). Typically, the chips are located on a ceramic substrate that serves several functions, one of which is to provide a structural platform to position and hold the chips. The substrate also provides for the electrical interconnections between the components via patterned conductive traces. In addition, it serves as a thermal connection to the outside world to dissipate heat from the operating device, via its attachment to the hybrid package.

The methods of attaching the components to the sub-strate and attaching the substrate to the hybrid package entail the use of either inorganic or organic "attach" materials. Inorganic attach materials include solder, gold-silicon eutectic, and silver glasses. Organic attach mate-rial s are usually epoxy-based adhesives that contain various types of fillers such as alumina, silver, or gold to provide control over the thermal and electrical char-acteristics of the attachment material. It is surprising to many that the attachment process, which is seemingly straightforward and uncomplicated, is actually the source of many difficulties in the operation of the hybrid device and consequently is the focus of much research.

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Most often , the attachment method of choice is the use of inorganic materials for both the attachment of the chip to the substrate and the substrate to the package, because the inorganic attach material s are chemically simple, clean, and stable. The procedure is usually straightfor-ward: simply heat to the attach's melting temperature, hold a few seconds to allow the material to flow, then cool. For relatively simple hybrid devices, this approach is quite adequate. As the device design becomes more complex, however, either through the need for a greater number of components per device or the need to use larger chips, problems begin to arise with the use of inorganic materials. Inorganic attach materials are me-chanically stiff and, when used with large chips, can impress a significant amount of stress on the chip result-ing from the difference in the coefficient of thermal expansion (CTE) between the chip and attachment mate-rial. This situation is also manifested when inorganic attach materials are used to attach the substrate to the package.

The fabrication of microelectronic hybrid devices is a multiple-step process that includes many sequences in-volving assembly, electrical testing, environmental stress-ing (time, temperature, and electrical power), and then electrical retesting . Because of the significant amount of work that must be expended to ensure the reliability of the devices, the overall cost of fabrication can be quite high. Consequently, if a problem is found during testing, the strategy is often to rework the device by removing the failed parts and replacing them with new parts, rather than discarding the device entirely. When a device fabricated with inorganic attach material s is reworked, it must be heated to the melting temperature of the attach material to remove the failed part. Consequently, the attachment material used on the other components may also melt, allowing those components to move and shift their align-ment on the substrate, which may introduce other prob-lems , in addition to problems associated with the device being repeatedly taken to rather elevated temperatures.

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When conditions are such that the inorganic attach materials are inappropriate for a particular fabrication process, the usual alternative is to use organic-based adhesives, which are typically epoxies. One important advantage of the organic adhesives is that the mechanical stiffness of epoxies is considerably less than that of the inorganic materials. Consequently, when an epoxy is used to attach items with largely different CTE'S, the ther-mally induced stresses that previously were applied to the chip or substrate and threatened its integrity are absorbed by the more pliable/elastic epoxy. Another advantage that the organic adhesives have over the inorganic attach materials is that a lower temperature is required to initiate the attachment process, typically 140C to 160C (cure temperature) versus 350C to 370C (melting tempera-ture); thus, rework can be performed at a much lower temperature. The device is brought up to a softening temperature, usually close to the cure temperature; the defective component is removed; then a new component and fresh adhesive are applied. With organic adhesives, unlike inorganic attach materials , no movement of the other components on the substrate occurs.

These advantages are accompanied by some disadvan-tages, however. Organic adhesives are chemically com-plex systems and much less stable than the inorganic attach materials. The complexity arises because organic adhesives consist of several materials with different re-sponsibilities and functions, and they react with one an-other as the material is cured. Two essential ingredients of the adhesive include the epoxy resin itself, which forms the backbone of the adhesive, and a curing agent, which is responsible for joining the epoxy resin subunits into a long extended ensemble. In all epoxy formulations, other material s are also present to modify the properties of the resin/curing agent mix. Generally, some type of filler is used; silver or gold can be added to make the epoxy mix electrically conductive, and an insulating filler such as alumina or silica can be added to provide better thermal conductivity and CTE. Other additives can include an accelerator, which is a compound designed to speed the action of the curing agent. An activator is sometimes present to activate the curing agent at the elevated curing temperature, so a less active curing agent can be used, allowing for a longer shelf life. Coupling agents , which are small bifunctional molecules that provide a chemical bridge between the epoxy, the filler, and the attachment surface, are usually present. Finally, solvents or reactive diluents are added to adjust the viscosity of the mixture.

During the cure process , many simultaneous and se-quential chemical reactions occur at different rates that depend on how fast the adhesive was brought to its cure temperature, the cure temperature itself, and the viscosity of the mix, which varies throughout the curing process. In addition, some secondary parameters can modify the effects of the primary parameters, such as the way the adhesive was deposited. For example, a thin film of epoxy sandwiched between two big surfaces will cure different-ly than a blob of epoxy at the edge of a chip (known as a fillet) . The variations in these systems are obviously quite great. Given the large number of constituents in the adhesive mix and the number of different reaction path-

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ways possible (intentional and unintentional) , it is not surpri sing that at the end of cure, a single, well-defined, homogeneous material is not produced. Rather, what one obtains is a heterogeneous collection of amorphous po-lymerized resin , randomly cross-linked by the curing agent, with voids containing trapped solvents or diluents , unreacted or partially reacted curing agents , accelerators, coupling agents, and condensation/degradation products. Also, the polymerized resin is not entirely inert, as it too can still possess reactive or partially reacted epoxy groups, hydroxyl groups, and other groups, which may undergo rearrangements over time to produce other by-products.

One of the more serious problems encountered with the use of epoxy adhesives is the release of materials from the bulk, usually occurring during the curing pro-cess, but also possible after cure. Material can escape from the bulk as a gaseous product, a process known as outgassing , or it can escape by diffusion along surfaces, a process known as bleed. Whether released by outgas-sing or bleed, the material will often find its way to other areas of the hybrid and may cause problems. Typical problems include wire-bond failures , where the impu-rity causes the metal-to-metal interface of the wire bond to be weak or more susceptible to other failure mech-anisms. I The deposition of polar chemical species onto and in between metal